In many communication systems, scheduling of users is performed from the network side, and is therefore sometimes referred to as network-based multi-user scheduling. For example, in previous generation systems, scheduling normally worked as an operation unit in the network controller. With the development of HSPA (High Speed Packet Access) and similar modern systems, scheduling was relocated to Node B.
For example, HSPA is generally based on High Speed Downlink Packet Access (HSDPA) in the downlink and Enhanced Uplink (EUL) in the uplink. The Enhanced Uplink (EUL) is sometimes referred to as High Speed Uplink Packet Access (HSUPA).
HSDPA is an enhancement to WCDMA (Wideband Code Division Multiple Access) that provides a smooth evolutionary path to higher data rates. HSDPA is specified in 3GPP release 5, and includes additional transport and control channels such as High-Speed Downlink Shared Channel (HS-DSCH). EUL is specified in 3GPP release 6 and includes additional transport and control channels such as the Enhanced Dedicated Channel (E-DCH).
Implementation of HSDPA enables improvements in capacity and end-user perception by means of efficient sharing of common resources in the cell among many users, rapid adaptation of the transmission parameters to the instantaneous radio channel conditions; increased peak bit rates and reduced delays. Fast scheduling is a mechanism that selects which user(s) to transmit to in a given Transmission Time Interval (TTI). The packet scheduler is a key element in the design of a HSDPA system as it controls the allocation of the shared resources among the users and largely determines the overall behavior of the system. In fact, the scheduler decides which users to serve and, in close cooperation with the link adaptation mechanism, which modulation, power and how many codes should be used for each user. This produces the actual end-user bit rate and system capacity. The HS-DSCH downlink channel is shared between users using channel-dependent scheduling to take advantage of favorable channel conditions in order to make best user of the available radio resources.
As mentioned, scheduling typically involves allocating communication resources to users according to some priority order. The scheduling algorithm generally determines the priorities of the user by using one or more metrics. Delay factors and optionally other factors based on radio channel quality are normally user to grant scheduling priorities to the users. For example, in MMTel services, the delay in the wireless access network is an important metric. It is known that so-called delay-sensitive schedulers for the downlink can achieve rather good performance for MMTel services such as Voice over IP traffic.
Similarly, to HSDPA in the downlink, there is a packet scheduler for E-DCH in the uplink. However, unlike HSDPA where the scheduler and the transmission buffer(s) are located in Node B, the data to be transmitted resides in the user equipment for the uplink case. The scheduler will normally operate on a request-grant principle, where the user equipment (UE) requests permission to send data and the scheduler on the network side decides when and how many terminals will be allowed to do so. A request for transmission will normally contain data about the state of the transmission data buffer and the queue at the terminal side and its available power margin. The standard foresees two basic scheduling methods, namely, long-term grants are issued to several terminals that can send their data simultaneously using code multiplexing, while short-term grants on the other hand allow multiplexing of terminals in the time domain.
In particular, for the Enhanced Uplink, the scheduler controls when and at which data rate the UE is allowed to transmit. By increasing the transmission power, the UE can transmit at a higher data rate. However, the received power from a certain UE represents interference for other UE terminals. Hence, the “shared resource” for EUL is typically the amount of tolerable interference in the cell. To control the uplink interference, the scheduler at Node B will allocate the UE with a value grant that corresponds to a maximum data rate.
The scheduler needs information about the UE status. Naturally, the more detailed the information, the better the possibilities for the scheduler to make accurate and efficient decisions. In EUL, there are two mechanisms for transferring scheduling information from the UE to NodeB: out-band signaling and in-band signaling. Out-band signaling is done through a single so-called “happy bit” transmitted on the Enhanced Dedicated Physical Control Channel (E-DPCCH). In-band signaling provides more detailed information and is transmitted on the Enhanced Dedicated Physical Data Channel (E-DPDCH).
The scheduler function for the enhanced uplink schedules EUL traffic of multiple users. EUL serves as a counterpart to the high-speed downlink packed access service in the WCDMA downlink. Together, EUL and HSDPA provide the backbone for the mobile broadband offer for the WCDMA cellular system. The scheduler operates in a closed loop fashion, where transmission grants (control signals) are issued in response to transmission requests and air interface load (measurements). The 3GPP standard provides channels with certain associated capacity, range, and delay properties. Notably, the control loop is dynamic, with nonlinear constraints and plenty of discrete ranges of various states.
The main problem with the existing scheduler functionality is that it does not account properly for the complicated multi-input-multi-output properties of the control loop, in fact the scheduler is designed without consideration of modern and systematic control theory, a fact that can be expected to lead to sub-optimal performance.
Therefore, there is a need for improved scheduling.